Can't use the content.dropboxapi

I want to know what i did wrong in this code. Because it is giving error. I am new to this so plz help.

Error: {"error_summary": "path/not_found/..", "error": {".tag": "path", "path": {".tag": "not_found"}}}

public class DropboxController
{
//Fetched from URL
String code;
String accesstoken;
public DropboxController()
{
code = ApexPages.currentPage().getParameters().get('code');
//Get the access token once we have code
if(code != '' && code != null)
{
AccessToken();
}
}

public PageReference DropAuth()
{
//Authenticating
PageReference pg = new PageReference('https://www.dropbox.com/1/oauth2/authorize?response_type=code&client_id=khvnmek12dw0wv9&redirect_uri=https://c.ap1.visual.force.com/apex/DropboxPage&state=Mytesting');
return pg;
}

public void AccessToken()
{
//Getting access token from dropbox
String tokenuri = 'https://api.dropbox.com/1/oauth2/token?grant_type=authorization_code&code='+code+'&redirect_uri=https://c.ap1.visual.force.com/apex/DropboxPage';
HttpRequest req = new HttpRequest();
req.setEndpoint(tokenuri);
req.setMethod('POST');
req.setTimeout(60*1000);

Blob headerValue = Blob.valueOf('khvnmek12dw0wv9' + ':' + '<REDACTED>');
String authorizationHeader = 'Basic ' + EncodingUtil.base64Encode(headerValue);
req.setHeader('Authorization', authorizationHeader);
Http h = new Http();
String resp;
HttpResponse res = h.send(req);
resp = res.getBody();

JSONParser parser = JSON.createParser(resp);
while (parser.nextToken() != null) {
if ((parser.getCurrentToken() == JSONToken.FIELD_NAME)){
String fieldName = parser.getText();
parser.nextToken();
if(fieldName == 'access_token') {
accesstoken = parser.getText();
}
}
}
system.debug('accessToken = '+accessToken );
System.debug(' You can parse the response to get the access token :: ' + resp);

string token = 'https://content.dropboxapi.com/2/files/download';
HttpRequest r = new HttpRequest();
r.setEndpoint(token);
r.setHeader('Authorization' , 'Bearer ' +accesstoken);
r.setHeader('Dropbox-API-Arg','{\"path\": \"/filepath\"}');
r.setMethod('GET');
r.setTimeout(60000);
Http h1 = new Http();
HttpResponse res1 = h1.send(r);
string resp1 = res1.getBody();
System.debug(' Information :- ' + resp1);
}
}